1. Field of the Invention
This invention relates to the field of human dietary supplements and more specifically to improved therapeutic products prepared from yeast and containing increased selenium concentrations with reduced toxicities.
2. Background
Selenium is an essential trace element for proper physiological function in humans. Decades ago, scientists demonstrated that selenium was incorporated into the chemical structure of an enzyme called glutathione peroxidase, an enzyme that is necessary to protect erythrocyte (red blood cell) cell membranes, and other biological structures against toxic reactions with highly reactive oxygen-derived species such as peroxides, and superoxides. The role of selenium in the biochemistry of glutathione peroxidase has been studied in some detail and the most current medical information confirms that trace amounts of selenium are required to maintain normal human health. Failure to ingest and absorb the necessary amounts of selenium can lead to improper functioning of the body's metabolic processes, and to various diseases and disorders.
Although many traditional nutrients, such as the natural occurring vitamins and minerals for which the U.S. Food and Drug Administration (FDA) has established a Recommended Daily Allowance (RDA), may be consumed in large quantities without adverse health effects, ingestion of high levels of some essential nutrients, such as certain metallic nutrients like selenium, may be toxic. To maintain ordinary health, one must balance the need for a minimum amount of such compounds with the need to protect against over-ingestion to the point of toxicity. For these compounds, ingesting low doses confers a significant nutritional benefit. However, when higher levels, i.e., amounts beyond the concentrations recognized as required for ordinary nutritional benefits, are ingested, beneficial health effects are not realized and the potential for dangerous toxicity exists.
Nutritionally beneficial quantities for daily doses for selenium have been found to be small. Nutritional selenium levels have been established by the FDA (see 21 C.F.R. 101.9(c)(8)(iv), January 1994). Humans and animals can safely metabolize limited amounts of both inorganic and organic forms of selenium and can convert non-methylated selenium to mono-ordi-or trimethylated derivatives, of which the monomethylated derivatives are most toxic. [Bedwal, R. S., et al., Medical Hypotheses, 41(2):150-159 (August 1993)]. The FDA has adopted Reference Daily Intakes (RDIs) of 70 micrograms for selenium. Selenium dosage of 600 micrograms per day has been reported as safe. [Ferris G. M. Lloyd, et al., App. Clin. Biochem.,26:83-88 (1989)]. At about this dosage, normal activity of the enzyme glutathione reductase safely converts selenogluthatione to hydrogen selenide in the liver and erythrocytes and is ultimately excreted. Thus, at such lower dosages, the body is able to safely metabolize and excrete selenium that is present in the free metallic form.
For many years, physicians and medical researchers have studied several potential health benefits resulting from the ingestion of low levels of selenium. For example, low concentrations of sodium selenate (an inorganic form of selenium) work with vanadium to improve glucose tolerance and to increase the levels of glucose-induced insulin release. However, as with many trace elements such as selenium, at higher dosage levels or concentrations, these beneficial effects are reversed and dangerous toxicity is manifested. [Furnsinn, C. et al., Internat'l J. of Obesity and Related Metab. Dis., 19(7):458-463 (1995)].
Therefore, the administration of selenium in the natural form involves a scientific and medical trade-off because, when administered in relatively low concentrations, selenium provides beneficial health effects, however, at higher concentrations, selenium exhibits dramatic toxicity such that the potential health benefits are lost and toxicity becomes the primary concern. This trade-off is particularly problematic when selenium administration is attempted, not as a dietary supplement, but rather in the treatment of disease. However, if the toxicity problems could be overcome, increased dosages of selenium could offer substantial advances in the treatment of several important disorders that affect human health. For example, the role of selenium in maintaining the function of the enzyme glutathione peroxidase has led researchers to examine the role of selenium in several disease states. In cancer, animal studies have shown that selenium protected against chemicals and ultraviolet energy sources known to cause cancer in humans. Selenium is believed to reduce the risk of certain cancers due to its properties as a strong antioxidant. A clinical study of more than 1,300 people found that those who took a daily supplement of selenium cut their overall cancer risk by nearly 40%. [Terence Monmaney, Selenium May Fight Cancer, Study Shows, LOS ANGELES TIMES, Dec. 25, 1996, at A 1, A 29]. In addition, U.S. Pat. No. 4,599,234 teaches that a combination of a selenium species (either organic or inorganic forms) with beta-carotene and a hydroxytoluene source significantly reduced the mortality of mice that were fed carcinogens and that these effects were better than those observed for the mice that were administered either selenium or beta-carotene or hydroxytoluene. U.S. Pat. No. 4,564,634 teaches a selenium-based nutritive composition having anti-neoplastic activity in which the selenium compound is used is a novel form of selenium prepared by a reaction of selenium metal with Tung oil (9,11,13-octadecatrienoic acid). [Schrauzer, G., Inorg. and Nutr. Aspects of Cancer, p. 330 (New York: Plenum Press (1978))].
Heart disease has also been shown to be reduced in persons who consume recommended amounts of selenium in their diet. In certain studies, the levels of selenium in the blood stream have been directly correlated with the degree of progression of cardiovascular disease with those patients having the lowest levels of selenium having the most extensive coronary artery blockage. In such cases, the glutathione peroxidase enzyme is thought to exert an antioxidant effect that protects the coronary vessels from disease. In a similar mechanism, selenium is thought to interact with prostaglandins to control free radical cascades that lead to elevated levels of prostaglandins and inflammation. Patients suffering from arthritis have been shown to have low levels of plasma selenium and their clinical condition has improved with dietary supplementation of selenium.
The precise mechanism by which selenium may protect from cardiovascular disease is not known, however, free radical antioxidant "scavengers," such as selenium, are believed to react with oxidants such that the oxidants are not available to form oxidized low density lipoproteins (O-LDLs). Thus, a reduction in the oxidants lowers the risk of arterial plaque deposits in blood vessels. Arterial plaque is precipitous material formed chiefly of oxidized low density lipoproteins (O-LDLs). The buildup of plaque in the form of O-LDL in the arteries is understood to be a factor in ischaemic heart disease. Free radical oxidants, many of which come from naturally occurring sources such as sun exposure, metabolism of certain nutrients, and exercise, act to oxidize low density lipoprotein (LDL) into its deleterious form, O-LDL. In contrast, high density lipoprotein (HDL) is understood to have beneficial health effects in the body. HDL is understood to be a more soluble form of lipoprotein, and its presence is not known to significantly contribute to the formation of arterial plaque. Since selenium functions to reduce the levels of O-LDL and thereby increase the level of HDL in the body, adequate quantities of selenium may decrease the likelihood of cardiovascular disease as well.
Based on the foregoing, increased concentrations of selenium are potential treatments for a variety of disorders as long as the selenium concentrations do not reach toxic levels. For this reason, several different forms of selenium have been investigated to determine the optimal form for administration to humans, either as a dietary supplement or as a therapeutic product for the treatment of disease. Yeast-derived selenium has been shown to be a less toxic form of selenium, and thus a preferred source of a selenium composition for human consumption. The selenium produced by yeast cultures undergoes a type of biosynthesis whereby inorganic selenium salts are converted to an organic form via intracellular incorporation into the yeast. These organic, biosynthesized selenium yeast derivatives are better nutritive sources of selenium because they are less toxic and more easily metabolized by the mammalian system than their inorganic counterparts. One method of producing a selenium-enriched product using food yeast such as Saccharomyces cerevisiae or Candida utilis has been reported. When dried and fed to rats, these selenium-enriched yeast effectively prevent hepatic liver necrosis. [Reed et al., Yeast Tech., AVI Publ. Co., Conn. (1973)]. Unfortunately, this method results in the production of a yeast product having a low intracellular selenium content, as well as a relatively high extracellular concentration of inorganic selenium.
Generally, high extracellular concentrations of selenium are to be avoided, while higher intracellular concentrations are preferred because this tends to indicate an increased relative concentration of selenium in the organic form which, as noted above, is preferred for administration to humans. For this reason, prior efforts at producing selenium-based yeast products have focused on the ability to provide increased intracellular concentrations of selenium. For example, U.S. Pat. No. 4,530,846 ('846) describes a method for producing a selenium-enriched yeast that yields yeast with a moderately high intracellular selenium content. The yeast produced by this method are cultivated using a procedure that involves incremental feeding of the yeast culture. With respect to the process of the '846 Patent and the limitations on selenium concentration using that method, the '846 patent states: "While intracellular selenium contents of yeasts are preferably in a range of 1,000 ppm or more, even as high as 2,500 ppm, the process has, as its practical limitations, the capacity of the yeast to assimilate the selenium during the yeast growth cycle without adverse effects on yield due to the selenium additive to the nutrients." In addition to the recognized limitations on the ability to achieve higher concentrations of intracellular selenium, the prior art also demonstrates that the existing yeast-derived selenium products still exhibit substantial toxicity. For example, the LD.sub.50 for the yeast product described in the '846 patent is reported by the assignee to be on the order of 7 mg per kilogram. In practice, the LD.sub.50 rating for a product limits the amount that may be administered to a human as part of a nutritional program or as part of an overall therapy to treat a disease. A relatively high LD.sub.50 is particularly disadvantageous when physicians or researchers attempt to administer an elevated selenium dosage, i.e., several times that recommended for dietary supplementation, in the treatment of disease.
There remains a need in the art for a yeast selenium product that provides high concentrations of selenium, preferably the organic form fixed in an intracellular form, that exhibits the lowest possible toxicities when measured by LD.sub.50. Ideally, such a product would be provided by a method to produce selenium-enriched yeast that results in: (1) a high growth rate of selenium-enriched yeast; (2) selenium-enriched yeast with high intracellular selenium content; and (3) low toxicity.